In my relation with Evangelicalism, I have of necessity thought about ecclesiology in the aspect of inter-church relations. The doctrine of separation is normally associated with more Fundamentalist circles, yet does it mean that it does not exist in Reformed circles?
After much thought, I have come to realize that the doctrine of separation is more a function within Evangelicalism only because of how the movement construes the Church. In Evangelicalism, both the Old and the New and any version of it, the default setting is one of unity, and therefore the doctrine of separation is the severing of the unity that exists due to the compromise of the other party. In Reformed churches however, the default setting is one of separation. The denomination is meant to be the union of Christ's churches on earth. Therefore, what is required with other church bodies is ecumenical relations. In Evangelicalism, it is a separation of the "unity" that was. In Reformed ecclesiology however, the focus is on uniting with those that have been separated. Reformed ecclesiology, properly construed, does not need a doctrine of separation per se since the default is separation. It sounds unloving, but properly construed Reformed ecclesiology as a matter of principle starts by declaring all churches not in communion with the Reformed Church(es) as being de facto and de jure separated from the Visible Church.
In the modern times of course, there needs to be an understanding of the doctrine of separation, not as an addition to Reformed ecclesiology, but as a foil to understand Reformed ecclesiology as opposed to Evangelical forms of ecclesiology. We are not Evangelicals, and the more I think and mature on the issue, the more I would not consider myself to be one.